Sunday, July 28, 2013

Animals (kingdom Animalia) are multicellular organisms that get the energy they need by consuming food. They are eukaryotes, i.e living things with cells containing a nucleus and other structures enclosed within membranes. They, however, lack rigid cell walls. All animals are able to move at least some part of their body, and many are able to move from place to place.

The various species of animals can be scientifically classified into smaller groups (called phyla) based on their physical appearances and, more recently, their DNA as well. A wide variety of animals from various phyla have been recorded from Singapore, and the members of the same phyla usually share a few physical or behavioural adaptations.

Here are some of the distinguishing features that will be useful in helping to make an educated guess of the phylum of an unfamiliar animal encountered in Singapore.

Hard corals - Some of the colourful sponges are often mistaken to be hard corals. Hard corals, however, have hard, rock-like skeletons, while sponges have a spongy or rubbery feel when touch. Avoid touching sponges with your bare hands though (wear gloves!), as many species have sharp spicules.

Soft corals - Some sponges with leaf-like or tree-like growth forms may be mistaken for soft corals. Sponges tend to be covered in tiny pores of varying sizes, but not soft corals.

Bryozoans - Encrusting or lace-like bryozoans may be mistaken for sponges. A closer examination will reveal cell-like patterns in the bryozoans, which are the zooecia (or exoskeleton) of the individual zooids (the bryozoan animal).

Tunicates - Colonial tunicates are often mistaken for sponges. To differentiate them from the latter, take a closer look to spot the individual zooids that will not be present in sponges.

Many of free-swimming and non-swimming ones come with a pair of long tentacles fringed with smaller tentacles. See this photo from Wikimedia which features numerous flatworm-like ctenophores extending their long tentacles on a sea star.

Most free-swimming species have eight strips (called comb rows) running down the length of their body. Each strip bears bands of hair-like projections (or cilia) which resemble a comb.

Sponges - Some sponges, depending on the growth form, may be mistaken for branching, boulder or encrusting hard corals and blue corals. Some may also be confused with soft corals. Sponges can be distinguished by the numerous tiny pores of varying sizes and the lack of a rock hard skeleton (to differentiate from hard corals).

Comb Jellies - Scyphozoan jellyfish and hydromedusae may be confused with comb jellies, but the latter only have two tentacles (if present) fringed with smaller tentacles. Most comb jellies also have eight strips (called comb rows) running down the length of their body.

Fan worms - Fan worms are sometimes mistaken for sea anemones and tube anemones due to their flower-like appearances. The tentacles of fan worms, however, are feather-like (with a main "stalk" in the middle and fine hairs lining the sides.)

Horseshoe worms - Horseshoe worms may also be mistaken for sea anemones, or a cluster of them may be mistaken for zoanthids or corallimorphs. Their crown of tentacles, however, are arranged in a somewhat horseshoe shape if viewed from the top, appearing like two separate spirals but are actually connected.

Feather stars - Due to the feather-like arms and pentaradial symmetry, feather stars may be mistaken for sea anemones by some people. They, however, lack the body column, and have claw-like arms on the underside.

Seaweed - Soft corals and hydroids may have plant-like appearances, but they lack the succulence of the latter. Also, the individual polyps will be radially symmetrical, and if they have retracted, there will be small bumps or holes on the surface of the colony housing them.

Fan worms - Fan worms are sometimes mistaken for horseshoe worms due to their flower-like appearances. The crown of tentacles of the horseshoe worm, however, is arranged in a somewhat horseshoe shape when viewed from the top, appearing like two separate spirals but are actually connected.

Cnidarians - Due to the crown of tentacles, and their habit of congregating sometimes, horseshoe worms may be mistaken for sessile cnidarians, such as sea anemones, tube anemones, zoanthids or corallimorphs.The crown of tentacles of the horseshoe worm, however, is arranged in a somewhat horseshoe shape when viewed from the top, appearing like two separate spirals but are actually connected.

The individual bryozoan animal (or zooid) usually lives in a "chamber" (or zooecium), which is a protective chitinous or calcareous exoskeleton.

A closer examination will usually reveal cell-like patterns in the bryozoans, which are the zooecia (or exoskeleton) of the individual zooids (the bryozoan animal). Some tree-like bryozoans (see diagram from bryozoa.net), however, may lack the cell-like patterns, and instead, the zooids are lined tightly along the sides of the upper branches.

The two valves are usually not symmetrical, but the line of symmetry is perpendicular to the hinge line.

Many species, such as the one recorded from Singapore anchors itself in soft substrates with a stalk-like structure called a pedicle.

May be confused with:

Bivalves - Lamp shells have a two-part shell, and hence are often confused with bivalves. Bivalves, however, have a ligament that joins the valves near the hinge, which forces the two valves open when the adductor muscles relax. Lamp shells lack this ligament, and hence their shells often remain close even after the animal is dead. And for bivalves, if a line of symmetry is present, it is usually parallel to the hinge line, while for the lamp shells the line of symmetry is perpendicular to the hinge line.

Have a distinctive balance sensory receptor (made up of a large cell bearing a calcareous body), appearing like a round spot near the front end. This is usually hard to see without a hand lens though.

The species recorded in Singapore are small (typically less than 0.5cm) and congregate on hard corals and corallimoprhs.

May be confused with:

Flatworms - Acoels appear like tiny flatworms, but they have a distinctive balance sensory receptor near the front end, appearing like a round dot when viewed under a hand lens. The species recorded in Singapore congregate on hard corals and corallimorphs.

Marine slugs - Some smaller sea cucumbers may be mistaken for marine slugs, but can be distinguished from the latter by having oral tentacles and tube feet.

Sea anemones - Feather stars are occasionally mistaken for sea anemones due to their long arms. They, however, lack the body column, and have claw-like arms on the underside. Some sea cucumbers, especially the burrowing ones, may be mistaken for sea anemones too when they extend their oral tentacles to feed. They have an anus though, which sea anemones lack.

Annelid worms - Some sea cucumbers may be mistaken for earthworms, peanut worms and other annelid worms due to their long and slim bodies, but can be distinguished by their oral tentacles and tube feet.

The body is very fragile and breaks apart when dug out from its burrow.

May be confused with:

Annelid worms - Hemichordates seen on the shore may be mistaken for earthworms, peanut worms and other annelid worms due to their long and slim bodies. The cast resembles those made by lug worms. The only sure way to distinguish hemichordates is to check for the three-part body plan.

Segmented species have a body comprising identical segments (excluding the head and tail) containing the same set of organs, and in some cases, external structures used for locomotion.

Unsegmented annelids are believed to have lost the segments through evolution, and include the spoon worms with a flattened proboscis resembling a spoon on their front ends, and the peanut worms with a body comprising an unsegmented trunk and a retractable structure called an "introvert".

May be confused with:

Hemichordates - Hemichordates may be mistaken for peanut worms and other annelid worms due to their long and slim bodies. The cast resembles those made by lug worms. The only sure way to distinguish hemichordates is to check for the three-part body plan.

Sea cucumbers - Some sea cucumbers may be mistaken for earthworms, peanut worms and other annelid worms due to their long and slim bodies, but can be distinguished by their oral tentacles and tube feet.

Chitons - Short-bodied annelid worms, such as scale worms, can be confused with chitons as both are usually found on rocky shores. Chitons, however, have an-eight-part shell on their back, while scale worms have numerous overlapping plated on their back.

Flatworms - Shorter ribbon worms may be mistaken for flatworms. Marine species are generally not paper thin like the marine flatworms, and terrestrial ribbon worms are more cylindrical than the terrestrial flatworms. In some cases, it can be extremely hard to distinguish them without looking at the internal parts.

Most marine species appear leaf-shaped, while terrestrial species recorded in Singapore have hammer-shaped heads.

May be confused with:

Acoels - Acoels appear like tiny flatworms, but they have a tiny distinctive balance sensory receptor (appearing like a round dot) near the front end when viewed under a hand lens, and the species recorded in Singapore congregate on hard corals and corallimorphs.

Ribbon worms - Shorter ribbon worms may be mistaken for flatworms. Marine species are generally not paper thin like the marine flatworms, and terrestrial ribbon worms are more cylindrical than the terrestrial flatworms. In some cases, it can be extremely hard to distinguish them without looking at the internal parts.

Annelid worms - Due to their segmented bodies, annelid worms are easily confused with arthropods that have elongate bodies, such as millipedes, centipedes, symphylans and caterpillars of moths and butterflies. Annelids, however, lack true segmented legs which the arthropods possess, and at most have bristle-like structures to aid with movement.

Velvet worms - Velvet worms are easily confused with arthropods with elongate bodies as well. They, however, lack the rigid exoskeleton of the arthropods, and have a pair of claws at the tip of each leg.

Molluscs - Barnacles may be easily confused with limpets, which also have conical shells. They are, however, permanently fixed on hard structures, while limpets are motile. Also, the wall of the barnacle's shell has numerous holes and gaps internally (much like a sponge!), holding many pockets of air for heat insulation. Barnacles may also be confused with sessile bivalves such as oysters, but the latter has a two-part shell.

Barnacles - Barnacles may be easily confused with limpets, which also have conical shells. They are, however, permanently fixed on hard structures, while limpets are motile. Also, the wall of the barnacle's shell has numerous holes and gaps internally (much like a sponge!), holding many pockets of air for heat insulation. Barnacles may also be confused with sessile bivalves such as oysters, but the latter have a two-part shell.

Scale worms - Short-bodied annelid worms, such as scale worms, can be confused with chitons as both are usually found on rocky shores. Chitons, however, have an-eight-part shell on their back, while scale worms have numerous overlapping plated on their back.

Sponges are very simple multicellular animals with no true tissue and organ. They also have no body symmetry, and are just congregation of several types of cells which form an organised structure with numerous pores and channels for the circulation of water.

Comb jellies are mostly free-swimming organisms that appear like jellyfish with eight strips running down the length of their bodies. Others creep over the substrate, resembling flatworms. Many comb jellies possess a pair of tentacles fringed with smaller tentacles.

Horseshoe worms are marine worms living in tubes and have a crown of tentacles arranged in a somewhat horseshoe shape if viewed from the top. One species, the Black Horseshoe Worm (Phoronis australis) which lives with tube anemones, can be seen in Singapore.

Bryozoans are tiny organisms that mostly live in colonies (except for one solitary species). Each individual bryozoan animal has a lophophore, which is a crown of tentacles for filter feeding, and lives in a chitinous or calcareous chamber-like exoskeleton (or zooecium).

Brachiopods, or lamp shells, are solitary, clam-like animals with a two-part shell. Unlike a clam which has a left valve and a right valve (based on how the valves are oriented to the body), a lamp shell has a dorsal (or "upper") valve and a ventral (or "lower") valve.

The acoels are flat, bilaterally symmetric, soft-bodied unsegmented worms typically found in the marine environment. They do not have a gut cavity, and have a distinctive balance sensory receptor (made up of a large cell bearing a calcareous body) at the front end.

Hemichordates are worm-like animals characterised by a three-part body - the front end, followed by a collar, and a posterior trunk. There is a flexible, hollow tube (the stomochord) in the collar region, somewhat resembling the notochord found in chordates.

Annelids are bilaterally symmetrical worms which may or may not be segmented. The segmented annelids will have a body comprising identical segments. Recent studies have shown that unsegmented worms such as the spoon worms and peanut worms are also annelids.

Ribbon worms are soft-bodied, bilaterally symmetrical animals that are mostly long, thin and flat (like a ribbon). Some species, however, have short and wide bodies. They have an eversible proboscis which can shoot out just above the mouth to capture/retrieve their food.

Flatworms are unsegmented worms with soft and bilaterally symmetrical bodies. They are mostly very flat, and hence the common name "flatworm". While many flatworms are parasites of other animals, we can still see many pretty free-living flatworms on our shores and forests.

Velvet worms are segmented, centipede-like organisms with numerous pairs of walking legs. Each leg has a pair of claws. They lack a rigid exoskeleton, and instead, the body cavity is filled with a fluid to make them firm. They hunt small invertebrates with a glue-like slime.

True mangrove species refer to those that grow only in mangrove environment. They are adapted to survive in saline, waterlogged and anaerobic conditions. Based on Tomlinson’s list, Singapore has 30 true mangrove species, and here's an ID key that I have developed to identify them.

These plants have each of their leaves fully subdivided into leaflets. Compound leaves can be differentiated from simple leaves by identifying where the petiole (the leaf stalk attaching the leaf to the stem) occurs. A new compound leaf also occurs as one with many small new leaflets.

The creepers and climbers featured here refer to plants with long and narrow stem that spread over the ground and lower structures (i.e. creepers) and those that climbing onto taller structures and trees (i.e. climbers), but exclude the grasses and grass-like plants.

An epiphyte is a plant that grows upon another plant. True epiphytes do not derive nutrients from the host plant, unlike parasitic epiphytes. Ferns are vascular plants which do not produce seeds but reproduce via spores. Herbs are flowering plant with no persistent woody stems.

Mistletoes are obligate hemiparasitic shrubs from the order Santalales. They have modified roots called "haustoria" which penetrate into the host plants' tissues to draw water and non-organic nutrients, but are able to photosynthesize to produce their own sugar.

The acoels, or acoel worms (phylum Acoelomorpha), are flat, bilaterally symmetric, soft-bodied unsegmented worms typically found in the marine environment.

They do not have a gut cavity, and hence the name "acoel", which means "no cavity" in Latin. Depending on the species, the mouth can be anywhere on the body, though for most species it is located in the middle of the underside. The ingested food is encompassed and digested by special digestive tissues. Some acoels feed on algae and detritus (i.e. tiny decaying organic particles), while others form symbiotic relationships with cnidarians and feed on any tiny organic matter trapped in the mucus produced by the latter, or harbour symbiotic algae which are able to photosynthesise and share food with the host acoels.

Acoels do not have excretory organs, and waste is usually excreted by some of the cells.

As they are mostly very flat, they were previously classified as flatworms (phylum Platyhelminthes). Studies have shown that they are more primitive than flatworms though, and can be distinguished from the latter by having a distinctive balance sensory receptor (or statocyst) at the front end. This statocyst is made up of a large cell bearing one calcareous body (or statolith) within a capsule formed by two cells. Sometimes, those that have been reproduced asexually may not have the statocyst though.

Despite the fact that they are very simple animals, acoels possess a very simple brain which may be shaped like a ring, a barrel or a two-lobed structure.

Acoels can reproduce sexually or asexually. They are simultaneous hermaphrodites (with both male and female reproductive parts), and in sexual reproduction, some may exchange sperms, while in other cases one may perform the role of a male while another the female. Some species have a specific opening for the female reproductive part, and for these species, the male reproductive part tends to be soft and muscular to be inserted into the female opening for sperm transffer. Other species may have female reproductive parts scattered around the body, and possess needle-like male reproductive parts to penetrate the skin of another acoel to inject the sperm. The fertilised eggs will then be laid. In asexual reproduction, they may split into two, or grow a new acoel from the body (this process is known as budding), which eventually detaches and develops into a separate animal.

Like many other simple animals, acoels are able to regenerate lost body parts.

The only acoel I have personally seen in Singapore are those that infest cnidarians. They are tentatively identified as Waminoa Acoels (Waminoa sp.). They are typically less than 0.5cm long.

Waminoa Acoels are known to harbour symbiotic algae which photosynthesise and contribute to the host acoel's nutritional needs. Their brownish coloration is probably due to the colour of the symbiotic algae. It is suggested that Waminoa Acoels feed on the organic particles trapped in the mucus produced by the host cnidarian to supplement their nutritional needs as well.

The Waminoa Acoels have been observed to infest many hard coral species, such as the favid coral (family Faviidae, probably Goniastrea spp.) above.

They also infest mushroom corals (family Fungiidae), such as the Podabacia sp. above.

While the acoels may not feed on the corals directly, it has been observed that some corals infested by them appear less healthy, such as the Sunflower Mushroom Coral (Heliofungia actiniformis) featured above. This coral appears sickly compared to the time when it was not yet infested.

The malacostracans (phylum Arthropoda, subphylum Crustacea, class Malacostraca) include the crustaceans that many people are familiar, such as crabs, lobsters, prawns and woodlice.

While they may be very diverse in their appearances, they have several common features. Their body typically comprises three main parts - a head with five segments, a thorax with eight segments, and an abdomen with six segments. The head may be fused with the thorax to form a cephalothorax. They often have a tough exoskeleton strengthened with calcium carbonate, and the carapace (if present) covers the gills but not the abdomen. They have prominent antennae, and up to three pairs of appendages may be modified into mouthparts for manipulating food.

As with other crustaceans, most malacostracans have two pairs of antennae and compound eyes on stalks. The abdomen ends off with a tail-like structure called a telson, and a flattened tail fan is usually present at the tip.

Most malacostracans reproduce sexually, though some are known to reproduce by parthenogenesis (the embryos develop without fertilisation). They usually have separate sexes, though a number of species are hermaphroditic (possessing both male and female reproductive parts), and some may change sex during the course of their lives. Most species carry the eggs until they are ready to hatch, though some release their eggs into the water. For aquatic species, the eggs will hatch into larvae that usually appear very different from the adults. For terrestrial species like the woodlouse, the young may appear similar to the adult but has fewer legs.

A wide variety of malacostracans can be found in Singapore, but only a few groups are readily seen and easy enough to distinguish from others. Hence, I will only include the few malacostracan groups that I have photographed in Singapore in this entry.

A) Mantis Shrimps (Order Stomatopoda)

Mantis shrimps (order stomatopoda) are elongate crustaceans with a pair of enlarged appendages that they carry folded beneath the head, somewhat similar to the forelimbs of a praying mantis. They can be divided into two main groups - the smashers and the spearers - based on the appearance of their striking appendages and how they are used. The smashers have a heavily calcified knob at the tip of the appendages for punching their prey, while the spearers have numerous sharps spines on the last segment of their claws to impale their prey.

Amphipods (order Amphipoda) are crustaceans that lack a carapace and often appear laterally compressed. They are very diverse but often overlooked due to the small sizes of terrestrial and intertidal species. Unlike many crustaceans, their eyes are unstalked, and in front of the eyes are two pairs of antennae. They typically have seven pairs of walking legs (pereopods), with the first two pairs modified to assist in feeding. They have an additional three pairs of swimming legs (pleopods) at the back, and three more appendages (uropods) to help with locomotion.

Isopods (order Isopoda) probably have the most diverse appearance among the crustaceans, ranging from flattened and bug-like, to longish and worm-like. Most of the ones seen on land and in shallow water have the former appearance though. They lack an obvious carapace, or have a much reduced one. With regards to their diet, some species are scavengers or detrital feeders, some are known to feed on algae, and some parasitise other animals, but generally, little is known about most isopods.

Decapods (order Decapoda) are very important to human, as many species are edible and important in the fishing and aquaculature industry. The body shapes of different groups differ greatly, from the elongate shrimps and lobsters to the short-bodied crabs. They can, however, be distinguished from other crustaceans by their five pairs of legs ("deca" means "ten", while "poda" means "feet") attached to the thorax. Some of these legs may have been modified to assist with feeding though, and some species may have additional appendages to help with locomotion.

Here are some of the decapods that I have photographed in local waters.

1. Prawns (Suborder Dendrobranchiata)

Prawns (suborder Dendrobranchiata) are distinguished from other similar-looking decapods by having gills that are branch-like. They also do not hold their eggs until they hatch, but release the fertilised eggs directly into the water column. While they are commonly called prawns in the region, they may be referred to as shrimps (not to be confused with the caridean shrimps) in other countries. They have an elongate body, and the first three pairs of legs have small pincers at the tip to assist with feeding. Most of the species seen in Singapore are of the family Penaeidae, and hence they are also often called penaeid prawns.

Members of this suborder typically have leaf-like gills or gills made up of unbranched filaments. They usually brood their eggs until they are ready to hatch.

Crabs (Infraorder Brachyura)

Crabs (infraorder Brachyura) can be recognised by their four pairs of walking legs, a pair of clawed arms, a broad carapace, and a very short and flattened abdomen which is usually folded underneath the body. The abdomen of females is usually broader, so as to carry the eggs. Their antennae are relatively short, and their eyes are on stalks. As this is a very huge group with many species, I will provide more details and photos of local examples in a separate page in the near future.

Anomurans (infraorder Anomura) include the hermit crabs, porcelain crabs, king crabs and their allies. They are often crab-like, but have long antennae and three or less pairs of obvious walking legs. The number of walking legs may not always be a useful feature though, as some true crabs have some of their legs modified for functions other than walking, and hence may have fewer than four pairs of walking legs. As there are a number of species in this group that I have photographed, I will provide more details and photos in a separate page in the near future.

Caridean shrimps (infraorder Caridea) appear very similar to penaeid prawns, with their elongate abdomen. Their five pairs of legs are all well-developed, but unlike the penaeid prawns, the third pair does not have pincers at the tip. Also, the second horizontal plate of the abdomen is enlarged, covering part of the first segment and the third segment. Unlike the prawns that release their eggs into the water, caridean shrimps brood their eggs under their abdomen until they hatch. As there are a number of species in this group that I have photographed, I will provide more details and photos in a separate page in the near future.

Mud lobsters & mud shrimps (Infraorder Gebiidea) are burrowing crustaceans with elongate and somewhat flattened abdomens that live mostly in soft-bottoms of intertidal or subtidal areas. The ones with a heavily calcified exoskeleton decorated with spines and tubercles are usually referred to as mud lobsters, while the more delicate members are referred to as mud shrimps. Some species may appear similar to the coral ghost shrimps of the infraorder Axiidea, but can be distinguished from the latter by having pincers on the first pair of legs, and very reduced or no pincers on the second pair, instead of having well-developed pincers on the first two pairs of legs. I will provide more details and photos in a separate page in the near future.

Spiny lobsters and allies (infraorder Achelata) are not true lobsters, and they can be distinguished from the latter by the lack of true claws with pincers. They have a soft and flexible tail, and a broad tail fan which allows them to swim rapidly backwards over a short distance. The rostrum (a pointy structure at the front of the head) is usually very small or absent. Many species are scavengers, though studies suggest that they may be opportunists which take a variety of food. While lobsters are occasionally seen by divers in local waters, some of the bigger specimens seen could be those released for religious merit-making. The above picture features a Painted Spiny Lobster (Panulirus versicolor) in the main image, and a dead Mud Spiny Lobster (Panulirus polyhagus) in the inset.